The FC package is an advanced packaging technique for connecting an integrated circuit (IC) die to a substrate, such as printed circuit board (PCB). During the packaging process, the IC die is turned upside down to connect to the IC die to the metal bond pads on the substrate and hence the metal conductive wires of the substrate.
The FC package generally comprises at least one substrate, a semiconductor die including a plurality of bumps, an underfill layer, and a plurality of solder balls. The substrate is commonly a dielectric material with two sides that have respectively a first metal interconnect layer including bond pads and a second metal interconnect layer. The first and the second metal interconnect layers respectively have a plurality of metal conductive wires located therein, and are electrically connected by a plurality of vias. The surface of the substrate generally includes a dielectric solder mask material in areas outside the bond pads. The solder mask over conventional copper traces prevents copper oxidation, masks against solder spreading around the solder joints and provides enhanced adhesion to the underfill. The circuit side (positive side) of the IC die has a plurality of die pads. Under bump metallurgy (UBM) is generally formed on the die pad surface before forming the bumps thereon.
The flipped IC die is generally bonded by soldering or an ultrasonic process (e.g. in the cases of Au—Au) to the bond pads of the first metal interconnect layer of the substrate (referred to as FC pads) through the bumps on the IC die surface. As used herein, any Au comprising stud to Au comprising surface interconnection is referred to as “Au—Au bonding”. In either case, the bonding is referred to as a “solder joint”. Then an underfill layer is formed in the gap region between the IC die and the substrate. Underfill generally comprises a polymeric material, such as a silica-filled epoxy resin. The function of the underfill is to reduce the stress in the solder joints caused by the coefficient of thermal expansion (CTE) mismatch.
“Soldering” is a first FC bonding method which generally refers to a thermal profile (reflow). This first bonding method does involve ultrasonic processing. A typical case for the first FC bonding method is for larger ASIC packages, which use solder on both the bumps of the IC die and on the FC pads of the substrate.
A second FC bonding method is Au—Au bonding which generally comprises a Au stud bump on the IC die that is attached to a Ni/Au surface on the surface of the FC pads of the substrate. This second bonding technique generally uses ultrasonic processing and thus does not require solder on the IC die or on the substrate.
There is third FC bonding method which uses a Au stud bump on the IC die which is bonded to solder finished FC pads. For this type of bonding although a Au stud on the IC die is used. This FC bonding is generally performed with thermal reflow such as described above for the first FC bonding method.
In general, for the first and third PC bonding methods the gap distance between bottom surface of IC die and top of substrate is large enough to not generally have significant problems in properly underfilling the gap region. The reason for this is because for solder comprising FC bonding, the IC die(s) tend to be larger, and for this reason the distance from center-to-center of the die pads is relatively large which allows the die-to-substrate connections to be larger, thus producing a larger top of soldermask to bottom of die space. This allows the soldermask layer to be placed on the substrate in the area under the IC die and still generally properly underfill the gap region with underfill.
However, in the second PC bonding method (Au—Au bonding), the solder joints are gold comprising including gold from both the bumped IC and gold from the flip chip pads on the substrate. Particularly in the case of FC Au—Au bonding technology, a very small gap results between the bottom of the die and the top of the substrate as a result of the Au stud collapse onto the EC pads on the substrate. With the solder mask layer on the substrate the gap between the IC die and the substrate for FC Au—Au bonding technology is generally about 5 μm. By not including a solder mask layer on the substrate the gap for FC Au—Au bonding technology can be increased to about 15 μm.